Half a million British, French, Canadian, Australian, New Zealand and German soldiers were killed or wounded in the four-month series of battles in 1917 called Passchendaele or, officially, Third Ypres. In this somber account of Allied attempts to achieve a breakthrough, Warner keeps his narrative firmly on the horror (``That men could survive such an experience and remain sane is, perhaps, more astonishing than the death toll''). A campaign of short attacks and counter-attacks on Belgian soil, Passchendaele was a struggle during which survivers became intimately familiar with details of terrain. This distinguished British historian, who has studied the ground closely, provides vivid ``landscape portraits'' that take on a haunting dimension in the final chapter, a guided tour of the battlefield. Unlike many historians who vilify the British commander-in-chief for ordering suicidal attacks, Warner ( Kitchener ) argues that he cannot name a contemporary general who could have done better at Passchendaele than Field Marshall Douglas Haig. Photos. (Oct.)